The compatibility packs should definitely be mandatory. And i'm so sick of "i don't want to play against him because hes not MK, it would ruin my fun" kind of crap. When you deny others the right to use a character they paid extra for, for YOUR fun, it takes away their fun. I'm sick of not being able to use my DLC online, its even stopped me from bothering to get better with them, because half of the people you run into online don't have the packs. At this point, its a situation of privileges. Should you have the privilege to say "No, i don't want to play against that character" or do those who pay the extra money have the privilege to be able to use their DLC at any time, in any online match. And since they paid, i personally believe you should be able to have your DLC characters accessed at anytime.